1. Field of the Invention
The present invention relates to a system for controlling a valve shift timing of an engine and, more particularly, to a system for controlling a valve shift timing of an engine, so adapted as to change the timing of opening or closing an intake valve or an exhaust valve in accordance with the running state of the engine.
2. Description of the Related Art
A system for controlling a valve shift timing of an engine is disclosed, for example, in Japanese Patent Unexamined Publication (kokai) Nos. 56-9,612, which comprises a rotatable cam arranged to rotate in synchronization with the rotation of the engine, a swingable cam disposed so as to be pivotally driven with the rotatable cam, and a locker arm arranged to lift a valve in association with the swingable cam. The surface of the rotatable cam with which the surface of the swingable cam is brought into contact is tapered in the direction of a rotational axis in which each of them rotates. Further, the valve-shift-timing control system has a drive means arranged so as to relatively move the swingable cam or the rotatable cam in its axial direction, thereby controlling changes of the valve shift timings by changing an initial phase of the swingable cam in accordance with the running state of the engine.
For such a conventional valve-shift-timing control system of the engine, the cam face of the swingable cam comprises an arc-shaped base section that does not lift the valve whatsoever and an arc-shaped lift section that lifts the valve in proportion to an amount of the pivotal movement of the swingable cam disposed adjacent to the arc-shaped base section. By relatively moving the swingable cam or the rotatable cam in its axial direction, the effective diameter of the rotatable cam varies to thereby change a valve lift amount, that is, a crank angle of the valve at which the valve is lifted or opened. In other words, in the state in which the swingable cam or the rotatable cam is relatively moved in its axial direction and the swingable cam is brought into contact with the larger diameter side of the rotatable cam, the effective diameter of the rotatable cam is increased and the swingable angle at which the swingable cam swings or pivots is increased to such an extent for the arc-shaped lift section to occupy all swingable angle. This arrangement provides the valve lift amount larger and the valve lift angle larger, at which the valve is lifted. On the other hand, in the state in which the swingable cam or the rotatable cam is relatively moved in its axial direction and the swingable cam is brought into contact with the shorter diameter side of the rotatable cam, the effective diameter of the rotatable cam is decreased and the swingable angle at which the swingable cam swings or pivots is decreased, thereby increasing the rate of the valve lift angle of the arc-shaped base section of the swingable cam relative to the entire valve lift angle of the swingable cam and increasing the valve lift amount. This arrangement makes the valve lift amount smaller and the valve lift angle smaller, at which the valve is lifted.
The conventional valve-shift-timing control system of the engine allows the valve lift amount and the valve lift angle to be adjusted on the basis of the relationship of the cam profile of the rotatable cam with the shape of the cam surface of the swingable cam; however, it is impossible to adjust the crank angle at which the valve is lifted to its maximal extent. In other words, the two valve lift characteristics can be gained in which the crank angles of the valve at which the valve reaches its maximal valve lift amount are the same, as shown in FIGS. 8(c) and 8(d); however, the two valve lift characteristics cannot be gained in which the crank angles thereof at which the valve reaches its maximal valve lift amount are different from each other, as shown in FIGS. 8(a) and 8(b). Hence, the conventional valve-shift-timing control system poses the problem that the extent of freedom is restricted in setting the valve characteristic.
Further, it is considered that the disposition of a means for making the shape of a cam surface of the swingable cam variable can offer two valve lift characteristics in which the crank angles at which the valve reaches its maximal valve lift amount are different from each other. However, the disposition of such a means makes the structure of the swingable cam complex, thereby leading to an increase in the inertia weight of the swingable cam and consequently decreasing the limit of rotation of the engine.